1. Field
Embodiments described herein relate to sense through the wall radar systems and in particular to systems for mitigating interference in sense through the wall radar.
2. Description of Related Art
Radar systems capable of sensing personnel through opaque barriers are of use to the military and to law enforcement. In a typical application, a radar unit may be deployed outside a building, and it may illuminate personnel, or targets, inside the building, with radio-frequency (RF) electromagnetic waves capable of penetrating the wall of the building. Reflections from the targets then return to the radar unit, passing through the wall again on their return, and are detected by the radar unit. The presence of human targets may then be inferred by a processing unit in the radar unit, and their locations may be communicated to the radar operator through an operator interface, which may include a graphical display.
Reliably identifying targets inside the building may be challenging because of multipath interference. For example, some of the radar radiation may reflect off of the wall, reflect from a target outside the building, and then reflect from the wall again, returning to the radar unit. This reflection from a target outside the building may be mistaken by the radar unit for, and incorrectly displayed to the radar operator as, a target inside the building. The problem of multipath may be exacerbated in sense through the wall applications by the attenuation caused by a wall, as a result of which the signal returning from a desired target inside the building may be weak compared to the signal returning from an undesired target outside the building.
Some undesired targets, both inside and outside the building, may be eliminated by suppressing stationary targets, using for example signal processing steps described in the '378 Application. Signals reflected from personnel inside the building may survive this suppression method even if the targets are intentionally standing still, because even a person attempting to stand perfectly still will move slightly as a result of heartbeat, breathing, and involuntary postural sway. Because these techniques suppress signals from stationary targets, they may not suppress multipath interference from undesired moving targets, such as personnel and wind-blown foliage outside the building.
A prior art approach to mitigating multipath interference involves equipping the radar unit with a rear-facing low gain receiving “guard” antenna. This antenna is more sensitive to reflections from undesired targets behind the radar than the main antenna, which is aimed into the building. Reflections detected by the main channel receiver which are also detected in the guard channel are then suppressed by the processing unit, so that they are not displayed to the radar operator. Although this approach helps to reduce the errors caused by multipath, its performance may be inadequate because reflections from inside the building may also reach the guard antenna, through a side lobe of this antenna or after reflection from the operator, resulting in the incorrect rejection by the processing unit of targets inside the building.
There is a need, then, for a system capable of reliably identifying multipath signals in sense through the wall radar systems.